Liquid crystal displays (LCDs) are optical displays used in devices such as laptop computers, hand-held calculators, digital watches and televisions. Some LCDs, for example in laptop computers, cell phones and certain LCD monitors and LCD televisions (LCD-TVs), are illuminated from behind using a backlight that has a number of light sources positioned to the side of the display panel. The light is guided from the light sources using a light guide that is positioned behind the display. The light guide is typically configured to extract the light from the light guide and to direct the light towards the display panel. This arrangement is commonly referred to as an edge-lit display, and is often used in applications where the display is not too large and/or the displayed image does not have to be very bright. For example, most computer monitors are viewed from a close distance, and so do not have to be as bright as an equivalently sized television display, which is typically viewed from a greater distance.
In larger, or brighter displays, the backlight tends to employ light sources positioned directly behind the display panel. One reason for this is that the light power requirements to achieve a certain level of display brightness increase with the square of the display size. Since the available real estate for locating light sources along the side of the display only increases linearly with display size, there comes a point where the light sources have to be placed behind the panel rather than to the side in order to achieve the desired level of brightness.
One important aspect of the backlight is that the light illuminating the display panel should be uniformly bright. Illuminance uniformity is particularly a problem when the light sources used are point sources, for example are light emitting diodes (LEDs). In such cases the backlight is required to spread the light across the display panel so that the displayed image has no dark areas. In addition, in some applications the display panel is illuminated with light from a number of different LEDs that produce light of different colors. It is important in these situations that the light from the different LEDs be mixed so that the color, as well as the brightness, are uniform across the displayed image.